Bucephalus had died before this, whilst Alexander was campaigning in India. Here is the account from The Anabasis of Alexander, by Arrian:

'Alexander founded two cities, one where the battle took place [The Battle of Hydaspes, against Porus], and the other on the spot whence he started to cross the river Hydaspes; the former he named Nicaea, after his victory over the Indians, and the latter Bucephala in memory of his horse Bucephalas, which died there, not from having been wounded by any one, but from the effects of toil and old age; for he was about thirty years old, and quite worn out with toil.'

Source: https://www.gutenberg.org/files/46976/46976-h/46976-h.htm#Page_295